Spinal fixation devices are used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires, or screws. The fixation rods can have a predetermined contour that has been designed according to the properties of the target implantation site, and once installed, the instrument holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
Spinal fixation devices can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a bone anchor with a threaded shank that is adapted to be threaded into a vertebra, and a rod-receiving element, usually in the form of a head having opposed U-shaped slots formed therein. The shank and rod-receiving assembly can be provided as a monoaxial assembly, whereby the rod-receiving element is fixed with respect to the shank, a unidirectional or uniplanar assembly, wherein the shank is limited to movement in a particular direction, e.g., within a single plane, or a polyaxial assembly, whereby the rod-receiving element has free angular movement with respect to the shank. In use, the shank portion of each screw is threaded into a vertebra, and once properly positioned, a fixation rod is seated into the rod-receiving element of each screw. The rod is then locked in place by tightening a set-screw, plug, or similar type of fastening mechanism onto the rod-receiving element.
In certain procedures, it is desirable to utilize a bone screw having a large diameter shank. Such large diameter bone screws often utilize a bottom-loading configuration, in which the head of the threaded shank is loaded into an opening in the bottom of the rod-receiving element. This can be done during manufacturing, or intraoperatively either before or after the threaded shank is implanted in bone. This allows the diameter of the shank to remain independent of the size of the opening formed in the rod-receiving element. However, the ability to perform correctional techniques with such bottom-loading bone anchor assemblies can be limited. These devices must be able to withstand correctional forces applied thereto when the rod-receiving element is angulated relative to the shank and/or during bending of a spinal fixation rod seated therein.
Accordingly, there remains a need for improved devices and methods for correcting a spine, and in particular to improved bottom-loading anchor assemblies and methods.